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Every living organism begins as a single cell and grows through a tightly regulated process called the cell cycle. In multicellular species, this cycle is essential for development, tissue repair, and reproduction. The cycle consists of two major phases: interphase and mitosis. Mitosis is the portion where the nucleus and its genetic material are duplicated and divided, producing two genetically identical daughter cells.

Interphase: Preparing for Division

Interphase is a continuous, non‑mitotic phase during which the cell grows, performs its specialized functions, and replicates its DNA. It is subdivided into three sub‑phases:

1. G1 (Gap 1) – The cell grows and synthesizes proteins necessary for DNA replication.
2. S (Synthesis) – DNA is replicated, doubling the chromosome count from 2N to 4N.
3. G2 (Gap 2) – The cell continues to grow and prepares the machinery needed for mitosis.

Mitosis: Four Key Stages

Mitosis is the division phase where the duplicated chromosomes are evenly distributed into two daughter nuclei. The process is traditionally divided into four stages, though an additional prometaphase can be recognized between prophase and metaphase.

1. Prophase

Lasting roughly 36 minutes in human cells, prophase marks the onset of visible chromosome condensation. The nuclear envelope dissolves, centrioles migrate to opposite poles, and spindle fibers begin to form.

2. Metaphase

Metaphase aligns chromosomes along the metaphase plate via spindle microtubules attached to kinetochores. This alignment ensures each daughter cell will receive an identical set of chromosomes. The stage typically lasts about 3 minutes.

3. Anaphase

During anaphase, spindle fibers shorten, pulling sister chromatids apart toward opposite poles. The centromere splits, and each chromatid becomes an individual chromosome, restoring the cell’s 2N chromosome count in each half.

4. Telophase

Telophase completes the division by re‑forming nuclear envelopes around the two chromosome sets, allowing chromatin to decondense. Cytokinesis usually follows, physically separating the cytoplasm and producing two distinct daughter cells.

Cytokinesis: Dividing the Cytoplasm

In animal cells, a contractile ring of actin filaments pinches the cell into two. Plant cells, with rigid walls, form a cell plate from vesicles that becomes a new cell wall, ensuring each daughter cell is fully separated.

Mitosis vs. Meiosis

While both processes involve chromosome segregation, their outcomes differ:

• Mitosis produces two genetically identical, diploid (2N) daughter cells, maintaining the organism’s chromosome number.
• Meiosis generates four haploid (N) gametes with half the chromosome number, enabling sexual reproduction and genetic diversity.

Understanding this distinction is critical for appreciating how organisms grow and reproduce.

Key Takeaways

• The cell cycle is a continuous, regulated sequence of growth and division.
• Mitosis ensures accurate distribution of genetic material to daughter cells.
• Meiosis introduces genetic variation essential for evolution.